Propagule banks, herbivory and nutrient supply control population development and dominance patterns in macroalgal blooms

Destructive macroalgal mass blooms threaten estuarine and coastal ecosystem s worldwide. We asked which factors regulate macroalgal bloom intensity, di stribution and species composition. In field experiments in the Baltic Sea, we analyzed the relative effects of nutrients, herbivores and algal propag ule banks on population development and dominance patterns in two co-occurr ing bloom-forming macroalgae, , Enteromorpha intestinalis and Pilayella lit toralis. Both species were highly affected by the combined effects of a propagule ba nk, herbivory and nutrients. The magnitude of effects varied with season. T he propagule bank was an important overwintering mechanism for both algae, and allowed for recruitment two months earlier than recruitment via freshly dispersed propagules. This provided a seasonal escape from intense herbivo ry and nutrient limitation later in the year. Favored by massive recruitmen t from the propagule bank, Enteromorpha was the superior space occupier in early spring, thereby reducing recruitment of Pilayella. Elimination of the propagule bank and recruitment via freshly dispersed propagules favored Pi layella. Strong and selective herbivory on Enteromorpha supported Pilayella in the presence, but not in the absence of the propagule bank. Nutrient en richment in summer counteracted herbivore pressure on Enteromorpha, thereby negatively affecting Pilayella. Herbivore and nutrient effects were more p ronounced for early life stages than adult algae. These results show that recruitment processes and forces affecting early li fe stages at the beginning of the vegetation period determine development a nd dominance patterns of macroalgal blooms. Herbivores naturally suppress b looms but increasing nutrient enrichment can override this important contro l mechanism. The propagule bank plays a previously unrecognized role for po pulation and community dynamics.